The problem with plastic wastes is highlighted these days. Plastic wastes such as wrapping materials are, after used by users, discarded and incinerated, or disposed of for land reclamation, etc. However, when such plastic wastes are incinerated, the heat of combustion is high and there occurs a problem in point of the curability of incineration furnaces. Polyvinyl chloride and the like give harmful gas and cause a pollution problem. Further, when they are disposed of for land reclamation, the shaped plastics do not degrade and semi-permanently remain as refuse that has the original shape, and their influence on the natural environment is problematic. Given that situation, various biodegradable plastics that are completely consumed by microorganisms in the natural environment to degrade into natural side products of carbon dioxide and water have been invented and have now amounted to the level of practical use.
For such biodegradable plastics, known is a composition of polylactic acid or thermoplastic polymer that comprises, as the essential ingredients, lactic acid and any other hydroxycarboxylic acid. For example, it is known that they have high mechanical strength and can be worked into containers and films of practical durability. However, the polymer has a high modulus of elasticity and has high toughness, but its flexibility is low. Therefore, it is unsuitable to applications of soft film of polyethylene, polypropylene, polyvinyl chloride, etc.
For softening resin, generally known is a method of adding plasticizer to it. Studies of using various types of plasticizer for softening resin have heretofore been made. For example, JP-A2000-302956 (EP1029890A) describes an aliphatic polyester composition that comprises an aliphatic polyester and, as a plasticizer, at least one compound selected from compounds (A) of the following formula: wherein at least one of R1, R2 and R3 is an acyl group having from 6 to 18 carbon atoms, and the remainder is a hydrogen atom or an acetyl group,and compounds (B) of a reaction product of a condensate of from 2 to 10 molecules of glycerin with a carboxylic acid having from 6 to 18 carbon atoms.
This document says that specific examples of the compounds (A) include glycerin diacetomonocaprylate, glycerin diacetomonolaurate and glycerin diacetomonooleate, and the compounds (B) are obtained through reaction of one mol of a glycerin condensate with from 0.8 to 1.2 mols of a carboxylic acid having from 6 to 18 carbon atoms, and their specific examples include tetraglycerin monocaprylate, decaglycerin monolaurate and decaglycerin oleate.
This document further says that the aliphatic polyester film obtained from the composition has excellent heat resistance and flexibility and no plasticizer migration of the film.
The film that comprises the compounds serving as plasticizer has excellent heat resistance and flexibility and no plasticizer migration of the film, but a film having high adhesion force to objects such as glass containers wrapped with it is desired.